The invention relates to a process for extracting xylitol from the end syrups of the xylitol crystallization.
Presently xylitol recovery is predominantly carried out by subjecting suitable xylaneous raw materials to an acid hydrolysis and, thereupon, extracting with water the sugar that was generated, by purifying and fully desalting the aqueous hydrolysates which then are hydrogenated, whereupon, following new desalting and concentration by evaporation, the xylitol is crystallized in one or more steps out of the water or else obtained by precipitation-crystallization using organic solvents (methanol, ethanol, propanol).
The end syrup recovered in the last crystallization stage is in the form of a mother liquor in a proportion of 5 to 10% by weight based on the starting raw material and still containing from about 30 to 60% xylitol in the dry substance.
It was previously believed that the further crystallization of the xylitol out of the final syrup was or would be prevented mainly by said syrup's content of arabinitol, sorbitol, galacitol, mannitol and other monomeric sugar alcohols, and accordingly processes have been developed which allow separating the monomeric sugar alcohols. Such procedures are described for instance in German Offenlegugnsschriften 2 418 801 and 2 710 374. The latter Offenlegungsschrift explicitly stresses that it is much easier to separate the monomeric sugar alcohols using weakly cross-linked, strongly acidic cation exchangers in the strontium-, aluminum- or iron-salt form than when using such exchangers in the form of calcium salt.
Now, it has been surprisingly found that the crystallization of the xylitol present in the end syrups is not so inhibited by the presence of other monomeric sugar alcohols than by the presence of oligo- and poly-saccharides (xylanes) and of hydrogenated oligo- and poly-saccharides (polysaccharide alcohols) which increase the viscosity of the end syrup so that--due to the decreased rate of diffusion for xylitol within the solution and the blocking of the xylitol integration into the crystal lattice--the crystallization will stop.